1. Field of the Invention
The present invention relates to a voltage measurement apparatus which measures the voltage of a battery-cell group that is made up of a plurality of battery cells connected in series.
2. Description of the Background Art
In recent years, in an electric car, a hybrid car, an electrically-driven tool or the like, an electrically-driven motor has been widely used as a mechanical power source or an auxiliary mechanical power source. As a power supply source to an electrically-driven motor, a battery-cell group formed by a secondary battery has been popular which is made up of a plurality of battery cells connected in series. For example, as such a battery-cell group, there is a 1.2-volt nickel-hydrogen battery, and a battery-cell group formed by connecting 240 cells in series is well known. In this battery-cell group, each battery cell has various temperature characteristics or capacitances. Hence, in the process of a charge or a discharge, an overcharge or an over-discharge may be generated in some cells. In order to prevent such an overcharge or over-discharge, it is essential to measure not only the voltage of a battery-cell group as a whole, but also the voltage of individual battery cells or each battery-cell block made up of a predetermined number of cells.
As a prior art of measuring the voltage of each battery cell which makes up a battery-cell group, for example, a flying capacitor circuit is known which is described in Japanese Patent Laid-Open No. 2001-201522 specification. FIG. 9 is a circuit diagram, showing the configuration of the flying capacitor circuit according to the prior art. As shown in FIG. 9, this flying capacitor circuit includes: a battery-cell group 100 which is formed by connecting N battery cells in series; cell-selection switch groups 200, 300 which choose any battery cell from the battery-cell group 100; a sampling switch 400; a transfer switch 600; an A/D converter 700.
A basic operation of the flying capacitor circuit will be described using FIG. 9. Specifically, the cell-selection switch groups 200, 300 choose any battery cell included in the battery-cell group 100. Then, the sampling switch 400 operates so that a capacitor 500 is charged by the chosen battery cell. After it has been charged, the sampling switch 400 is turned off and the transfer switch 600 is turned on. Thereby, the voltage of the charged capacitor 500 is transformed into a digital value by the A/D converter 700. Consequently, the chosen cell's voltage can be obtained by the sampling switch 400.
Herein, an electric switch is each used as the above described cell-selection switch groups 200, 300, sampling switch 400 and transfer switch 600. This electric switch is sure to have a parasitic capacitance. Hence, when the capacitor 500 is charged by any battery cell, the parasitic capacitance affects the capacitor 500 and produces a voltage error in a measurement voltage which is the voltage of the capacitor 500 after charged. According to the prior art, a disadvantage arises in that among such parasitic capacitances, a ground capacitance Cs may generate a voltage error. This voltage error ΔV is given in the following expression (B).ΔV=ΣCs/C×V(n−1)  (B)
Herein, reference character Cs denotes a ground capacitance, and ΣCs designates the sum total of (N+1) ground capacitances in all which are connected to the upper electrode of the capacitor 500. According to the prior art, the sampling switch 400 operates so that the capacitor 500 is charged by the chosen battery cell. After the sampling switch 400 has been turned off, a lower transfer switch 602 is turned on, so that the capacitor 500's ground potential falls to a grounding voltage. Sequentially, an upper transfer switch 601 is turned on, and thereby, the capacitor 500's voltage is transferred to the A/D converter 700. This helps resolve the disadvantage in that a ground capacitance may cause a voltage error.
However, in the flying capacitor circuit, as the cell-selection switch groups 200, 300 and the sampling switch 400, a photo switch which requires no drive bias is frequently used, such as a photo coupler and a photo MOS FET. In this photo switch, the grounding capacitance is small, specifically, approximately 0.5 pF, but the inter-terminal capacitance is large like 30 to 50 pF. Hence, rather than the grounding capacitance, the inter-terminal capacitance would affect the voltage error. Specifically, when the cell-selection switch groups 200, 300 and the sampling switch 400 have been turned on, the electric charge stored by the inter-terminal capacitance while being turned off moves to the capacitor 500. Thereby, a voltage error takes place in the capacitor 500.
Furthermore, as the capacitor 500, if a capacitor which has a capacitance of about 0.2 μF is used, the voltage error caused by the ground capacitance can be narrowed to such a degree that no problem could practically be raised.
Moreover, when the cell-selection switch groups 200, 300 which choose a battery cell targeted for measurement have been turned on, the transfer switch 600 is kept turned off. Hence, this switch's inter-terminal capacitance prompts the turned-on cell-selection switch groups 200, 300 to connect the battery-cell group 100 and the A/D converter 700 so that an electric current alternates. When an electric current passes between the battery cell targeted for measurement and the capacitor 500, an over-voltage caused by this battery cell's voltage is applied to the A/D converter 700. In some cases, the A/D converter 700 may be destroyed.